When an electron beam accelerated to about the speed of light in vacuum is bended in a magnetic field, radiation is emitted in a tangential direction of a traveling track of the electron beam and this is called a synchrotron radiation. The study for practical use of various techniques using characteristics such as high orientation, high intensity, high polarization by setting a light source that generates this synchrotron radiation at a straight section of an electron storage ring. In the electron storage ring of nowadays, many undulators that are high-intensity light sources each having a smaller beam section and higher beam directivity are provided.
This undulator adopts a construction in which a first magnetic circuit and a second magnetic circuit are oppositely arranged through a space to form a periodic magnetic field through which an electron beam passes as shown in the following Japanese Unexamined Patent Publication No. 2000-206296 or In-vacuum undulators of SPring-8, T. Hara, T. Tanaka, T. Tanabe, X. M. Marechal, S. Okada and H. Kitamura: J. Synchrotron Radiation 5, 403 (1998) and Construction of an in-vacuum undulator for production of undulator x-rays in the 5-25 KeV region. S. Yamamoto, T. Shioya, M. Hara, H. Kitamura, X. W. Zhang, T. Mochizuki, H. Sugiyama and M. Ando; Rev. Sci. Instrum. 61 (1992) 400. In order to generate the periodic magnetic field, each of the first and second magnetic circuits includes many arranged permanent magnets. When a strong magnetic field is to be generated, the first magnetic circuit and the second magnetic circuit are to be brought close to each other, so that an interval (gap) of the space can be narrowed and the magnetic field is intensified. Thus, a construction in which the first magnetic circuit and the second magnetic circuit are contained in a vacuum chamber is adopted. In this construction, there is an advantage such that the gap can be narrowed as compared with a construction in which a vacuum chamber is provided between a first magnetic circuit and a second magnetic circuit.
However, even when the gap is narrowed as described above, there is a limit in characteristics of the permanent magnet. In addition, when the gap is narrowed too much, there arises a new problem such that the permanent magnet is demagnetized due to radiation generated when the electron beam impinges on the permanent magnet. Therefore, there is a limit of intensifying the magnetic field only by a method of narrowing the gap.